Engine systems may be configured with exhaust gas recirculation (EGR) systems via which at least a portion of the exhaust gas is recirculated to the engine intake. Various sensors may be coupled in the engine system to estimate the amount of EGR being delivered to the engine. These may include, for example, various temperature, pressure, oxygen, and humidity sensors. Since the accuracy of the EGR estimation relies on the correct functioning of the various sensors, periodic sensor diagnostics are used. Further, engines with or without EGR need an estimate of air dilution to optimally set the ignition timing, among other controls. Combustion air dilution may be determined based on humidity measurements using humidity sensors, for example.
One example approach for diagnosing a humidity sensor is illustrated by Xiao et al. in U.S. Pat. No. 7,715,976. Therein, humidity sensor degradation is determined based on a comparison of an intake humidity estimated by a first humidity sensor in the intake manifold with an exhaust humidity estimated by a second humidity sensor in the exhaust manifold and an ambient humidity estimated by a third humidity sensor located outside of the engine. The sensor readings are compared during conditions when all the sensor readings are expected to be substantially equal, such as during engine non-fueling conditions in which the EGR valve is closed. If the readings of the three humidity sensors differ by more than a threshold, humidity sensor degradation may be determined.
However, the inventors herein have identified a potential issue with such an approach. The accuracy of determining degradation of any one humidity sensor may depend on the proper functioning of the other humidity sensors. Further, multiple humidity sensors may not be needed for engine control, and thus additional humidity sensors may not be available for comparison.
Another example approach for diagnosing a humidity sensor is illustrated by Pursifull et al. in U.S. Pat. No. 9,482,172. Therein, humidity sensor degradation is indicated based on a comparison of a response of the humidity sensor to a first gas flow and a second gas flow having a known humidity, or at least where the second gas flow is known to have a significantly different humidity than the first gas flow. The second gas flow may include intake manifold air, EGR, exhaust gas, and/or crankcase gas, for example. If the humidity sensor output is different than an expected output when the sensor is exposed to the second gas flow, degradation of the humidity sensor may be indicated.
However, the inventors herein have identified a potential issue with such an approach. For example, such an approach may be susceptible to false indications of degraded humidity sensor(s) if a difference in humidity of the first gas and the second gas is not enough to trigger a significant response in the humidity sensor during the second gas flow as compared to the first. Furthermore, such an approach may rely on engine operation, which may be limited in the case of hybrid electric vehicles (HEVs), start/stop (S/S) vehicles, etc., thus decreasing opportunities for humidity sensor diagnostic tests.
Thus, the inventors herein have developed systems and methods to at least partially address the above-mentioned issues. In one example, a method is provided comprising obtaining a baseline humidity measurement via a humidity sensor positioned in an intake system of an engine, while rotating the engine unfueled in a reverse direction, injecting a fluid into an exhaust system coupled to the engine which is drawn by the engine into the intake system, and rationalizing the humidity sensor as a function of a humidity measurement of the injected fluid by the humidity sensor and the baseline humidity measurement. In this way, the humidity sensor may be rationalized while the engine is not being fueled, which may improve fuel economy and may result in an increased lifetime of the engine.
In one example, the fluid comprises diesel exhaust fluid.
In an example, obtaining the baseline humidity measurement comprises commanding a throttle positioned in the intake system to a predetermined open position to couple the intake system and humidity sensor to atmosphere just prior to rotating the engine unfueled in the reverse direction. In such an example, obtaining the baseline humidity measurement and rationalizing the humidity sensor via rotating the engine unfueled in reverse and injecting the fluid into the exhaust may be commenced responsive to a predetermined duration of time elapsing since a vehicle-off event, where rationalizing the humidity sensor is requested during the vehicle-off event.
In some examples, rotating the engine in reverse is conducted at a predetermined engine speed, and may further include rotating the engine in reverse for a predetermined duration which comprises an amount of time where a response from the humidity sensor is expected provided that the humidity sensor is functioning as desired, without routing the fluid to atmosphere.
In another example, rationalizing the humidity sensor further comprises indicating that the humidity sensor is functioning as desired responsive to the humidity sensor outputting a signal that is greater than a threshold amount above the baseline humidity measurement, and indicating that the humidity sensor is not functioning as desired responsive to the humidity sensor outputting a signal that is lower than the threshold amount above the baseline humidity measurement.
In still another example, such a method may further include responsive to rationalizing the humidity sensor, rotating the engine unfueled in a forward direction, to route the fluid from the intake system back into the exhaust system.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.